To elaborate on DeWitt, an l(0) process is white noise and an l(1) process, a classical random walk ("brownian motion"). l(2) is a bit harder to describe: perhaps the location of a vehicle tracked by an inertial device, where the acceleration is "noisy".

Taleb doesn't argue that outliers are impossible to quantify, he just thinks that the present ordinary brownian motion model underlying mathematical finance should be replaced with fractional brownian motion.